1. Field of the Invention
This invention relates to blades for a propeller fan which is driven at a relatively low speed for supplying air to automobile engines or other kinds of apparatus for cooling or other purposes, and which is made of, for example, a synthetic resin or metal.
2. Description of the Prior Art
A known propeller fan is partly shown in FIG. 6 or 7 by way of example. Both of the fans comprise a plurality of blades 12 projecting radially outwardly from the outer peripheral surface of an annular boss 11. Each blade 12 of the fan shown in FIG. 6 has a multiplicity of small apertures 14 formed therethrough between its longitudinally central portion and its rear edge 13. The apertures 14 allow air to flow therethrough and thereby reduce a difference of air pressure between the suction and delivery sides of each blade 12 in order to decrease the separation of air in a boundary layer from the surface of the blade on its suction side and thereby the noise of the fan. Each blade 12 of the fan shown in FIG. 7 has along its rear edge 13 a plurality of notches 15 which are provided for preventing the appearance of a large swirling flow of air in the vicinity of the rear edge 15 to reduce the noise of the fan.
The propeller fan produces a noise when it is driven. The noise is mainly caused by the sound of the separating air, the sound of the flowing air and the sound of the pitching air. The sound of the separating air is due to the appearance of a swirling flow of air as a result of the separation of air in the boundary layer from the surface of each blade on its suction side. There exist two flows of air along the surface of each blade on its suction side, i.e., a flow of air in an outer layer having a substantially constantly high velocity and a large amount of kinetic energy and a flow of air in an inner or boundary layer contacting the blade surface and having a low velocity and a small amount of kinetic energy which is due to the viscosity of air. The boundary layer has a thickness which increases with an increase in air velocity. The boundary layer grows and has an increased thickness toward the rear edge of the blade until it is eventually separated from the blade surface to form a swirling flow of air.
Neither of the blade constructions shown in FIGS. 6 and 7 is very effective for reducing the noise of the fan including the sound of the separating air. Moreover, the presence of the apertures or notches lowers the air supplying capacity of the fan, as well as the mechanical strength of the blades per se.